Step 1: Get the dog used to handling: Give praise and treats at the same time as running your hand over the dog's body. You can also do this while touching the paws, face, and tail. Let the dog pull away or leave if he chooses to.
Step 2: Introduce the tool: We used a brush in this example, but repeat the same exercise above, only use the brush instead of your hand.
Step 3: Increase duration: Begin to add time between touching the brush to the dog, and giving the reward. If your dog is showing any signs of stress (like yawning, lip licking, growling, etc), take a step back and decrease the criteria.
Remember to keep these sessions short - even just a few minutes at a time, to keep things interesting for the dog. Always end sessions on a high note, and set your dog up to succeed. If you find yourself, or your dog getting frustrated, take a break and try again later.